Apparatus and method for determining the relative position and orientation of neurostimulation leads

ABSTRACT

Interelectrode impedance or electric field potential measurements are used to determine the relative orientation of one lead to other leads in the spinal column or other body/tissue location. Interelectrode impedance is determined by measuring impedance vectors. The value of the impedance vector is due primarily to the electrode-electrolyte interface, and the bulk impedance between the electrodes. The bulk impedance between the electrodes is, in turn, made up of (1) the impedance of the tissue adjacent to the electrodes, and (2) the impedance of the tissue between the electrodes. In one embodiment, the present invention makes both monopolar and bipolar impedance measurements, and then corrects the bipolar impedance measurements using the monopolar measurements to eliminate the effect of the impedance of the tissue adjacent the electrodes. The orientation and position of the leads may be inferred from the relative minima of the corrected bipolar impedance values. These corrected impedance values may also be mapped and stored to facilitate a comparison with subsequent corrected impedance measurement values. Such comparison allows a determination to be made as to whether the lead position and/or orientation has changed appreciably over time. In another embodiment, one or more electrodes are stimulated and the resulting electric field potential on the non-stimulated electrodes is measured. Such field potential measurements provide an indication of the relative orientation of the electrodes. Once known, the relative orientation may be used to track lead migration, to setup stimulation configurations and parameters for nominal stimulation and/or navigation. Also, such measurements allow automatic adjustment of stimulation energy to a previously-defined optimal potential field in the case of lead migration or postural changes.

[0001] The present application claims the benefit of U.S. ProvisionalPatent Application Serial No. 60/338,331, filed Dec. 4, 2001, whichapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to neurostimulation systems, suchas a spinal cord stimulation (SCS) system, and more particularly to amethod for determining the relative position and orientation ofelectrodes on a neurostimulation lead or leads used with such a system.

[0003] In SCS systems, positioning of the leads is critical to thesuccess of the therapy. During surgery, the physician places the leadsin a very careful manner in order to locate the electrodes proximal toneural elements that are the target of the stimulation. During and afterplacement, stimulation energy is delivered to verify that the leads areindeed stimulating the appropriate neural elements.

[0004] However, if the leads happen to shift position, the targetedneural elements may no longer be appropriately stimulated. At best, thiscan require electrical reprogramming to restore therapy or, at worst,surgical revision, where a second surgery is required and the leads mustbe manually readjusted. In the first case, the physician may have only asuspicion that a lead has shifted position, based on patient reportingof paresthesia, which is not foolproof. Also, attempting to reprogramthe leads based on paresthesia locations can be challenging.

[0005] What is needed is a more objective technique for verifying theposition of the leads.

[0006] Prior art approaches for determining the lead position aredisclosed in U.S. Pat. Nos. 4,486,835; 4,539,640; and 5,184,624, whichpatents are incorporated herein by reference.

SUMMARY OF THE INVENTION

[0007] The present invention addresses the above and other needs byproviding a cross-check technique for verifying the position of theelectrodes of the implanted leads. A first technique involves the use ofinterelectrode impedance. A second technique involves measured fieldpotentials. Either technique advantageously allows the relativeorientation of one electrode on an implanted lead to other electrodes onthe implanted lead or adjacent implanted leads in the spinal column orother body/tissue location to be readily determined. Such techniques areuseful not only for reprogramming, but also to estimate if the shiftedorientation of the electrodes is sufficiently large so as to makeelectrical reprogramming a waste of time, thereby suggesting thatsurgery may need to be performed for repositioning.

[0008] At present, the correct lead position may only be determined byX-ray or fluoroscopy. Disadvantageously, X-ray and fluoroscopy requireexpensive equipment, significant time, and appropriate medicalfacilities, most of which are not readily available.

[0009] The general process for fitting a neurostimulation patient, i.e,a spinal cord stimulation patient, is described, e.g., in U.S. Pat. Nos.6,052,624; 6,393,325; in published international patent application WO02/09808 A1 (published Feb. 7. 2002); and in U.S. patent applications(assigned to the same assignee as the present application) Ser. No.09/626,010, filed Jul. 26, 2000; and Ser. No. 09/740,339, filed Dec. 18,2000, which patents, publication, and applications are incorporatedherein by reference. As indicated in those documents, prior to fitting apatient with the certain types of neurostimulation leads, the relativeorientation of the electrodes on the implanted leads should be known inorder to allow appropriate navigation of the stimulation energy. Atpresent, a determination of the relative orientation typically requiresthat a fluoroscope or X-ray image of the implanted leads be present atthe time of patient setup with the system programmer. Disadvantageously,however, such images may not always be available. Moreover, between thetime of implant and follow-up visits, the leads may have shifted and thefluoroscope image may no longer be valid. This can result in poorpatient outcomes due to inappropriate or unexpected stimulation effectsduring fitting.

[0010] Hence, it is seen that there is a need for the cross-checktechniques provided by the present invention, which techniques can beused to verify the position of the leads at numerous times during thelifetime of the implanted leads, e.g., during initial implantation andprogramming, during followup visits, throughout the trial period, andduring subsequent reprogramming sessions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The aspects, features and advantages of the present inventionwill be more apparent from the following more particular descriptionthereof, presented in conjunction with the following drawings wherein:

[0012]FIG. 1 illustrates a neurostimulation system wherein two leads,each having eight in-line electrodes thereon, are positionedside-by-side, and wherein each lead is connected to an implantable pulsegenerator (IPG), which IPG is, in turn, coupled to an externalprogrammer;

[0013]FIG. 2 shows a functional block diagram of an IPG that usesmultiple programmable current sources to activate selected electrodes ofthe neurostimulation leads;

[0014]FIG. 3 shows a functional block diagram of an IPG that usesmultiple programmable voltage sources to activate selected electrodes ofthe neurostimulation leads;

[0015]FIG. 4 is a table that contains impedance vector and distanceimpedance data in accordance with one embodiment of the invention;

[0016]FIG. 5 illustrates representative relative electrode orientationin a patient having dual quadrapolar leads (two side-by-side leads, eachhaving four in-line electrodes thereon);

[0017]FIG. 6 is an impedance map that illustrates application of oneembodiment of the invention to the electrode orientation shown in FIG.5;

[0018]FIG. 7 depicts a representative fluoroscopic image of dualquadrapolar leads in a patient;

[0019]FIG. 8 illustrates, in accordance with another embodiment of theinvention, the measured electrode potential of non-activated electrodeson the dual quadrapolar lead of FIG. 7 when the activated electrode isactivated through monopolar stimulation;

[0020]FIG. 9 illustrates the measured electrode potential ofnon-activated electrodes on the dual quadrapolar lead of FIG. 7 when theactivated electrodes are activated through tripolar stimulation; and

[0021]FIG. 10 is a flowchart that highlights the main steps used withone embodiment of the present invention.

[0022] Corresponding reference characters indicate correspondingcomponents throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The following description is of the best mode presentlycontemplated for carrying out the invention. This description is not tobe taken in a limiting sense, but is made merely for the purpose ofdescribing the general principles of the invention. The scope of theinvention should be determined with reference to the claim(s).

[0024] The present invention uses: (1) interelectrode impedance (onetechnique or embodiment) or (2) measured field potentials (anothertechnique or embodiment) to determine the relative orientation of oneelectrode on an implanted lead to other electrodes on the implanted leador adjacent implanted leads in the spinal column or other body/tissuelocation.

[0025] Before describing the two techniques, either of which may beused, it will be helpful to first briefly provide an overview of arepresentative neurostimulation system of the type with which thepresent invention may be used. A representative neurostimulation systemis illustrated in FIG. 1. Such system may include a first implantablelead 20 and a second implantable lead 30. Each lead includes a series ofin-line electrodes thereon. For the example shown in FIG. 1, the firstlead 20 contains eight in-line electrodes E1, E2, E3, . . . E8. Thesecond lead 30 also contains eight in-line electrodes E9, E10, E11, . .. E16.

[0026] Each of the electrodes of each lead 20 or 30 are electricallyconnected through respective wires, embedded or carried within a body ofthe lead, to an implantable pulse generator (IPG) 40. The wiresconnected to the electrodes E1, E2, E3 . . . E8 of lead 20, for example,may be characterized as a bundle of wires 22 that are electricallyconnected with the IPG 40. Similarly, the wires connected to theelectrodes E9, E10, E11, . . . E16 of lead 30 may be characterized as abundle of wires 32 that are electrically connected with the IPG 40.Through these wires, carried within the respective leads 20 or 30, theIPG is able to direct electrical stimulation to selected electrodes ofeach lead.

[0027] When a given electrode is selected to receive an electricalstimulus, it is (for purposes of the present invention) said to be“activated”. When an electrode is not selected to receive an electricalstimulus, it is said to be “non-activated”. Electrical stimulation mustalways occur between two or more electrodes (so that the electricalcurrent associated with the stimulus has a path from the IPG to thetissue to be stimulated, and a return path from the tissue to the IPG).The case of the IPG may function, in some modes of operation, as areturn electrode E_(R). Monopolar stimulation occurs when a selected oneof the electrodes of one of the leads 20 or 30 is activated along withthe return electrode E_(R). Bipolar stimulation occurs when two of theelectrodes of the leads 20 or 30 are activated, e.g., when electrode E3of lead 20 is activated as an anode at the same time that electrode E11of lead 30 is activated as a cathode. Tripolar stimulation occurs whenthree of the electrodes of the leads 20 or 30 are activated, e.g., whenelectrodes E4 and E5 of lead 20 are activated as an anode at the sametime that electrode E13 of lead 30 is activated as a cathode. Ingeneral, multipolar stimulation occurs when multiple electrodes of theleads 20 or 30 are activated.

[0028] The IPG 40 is typically programmed, or controlled, through theuse of an external (non-implanted) programmer 60. The externalprogrammer 60 is coupled to the IPG 40 through a suitable communicationslink, represented in FIG. 1 by the wavy arrow 50. Such link 50 passesthrough the skin 18 of the patient. Representative links that may beused to couple the programmer 60 with the IPG 40 include a radiofrequency (RF) link, an inductive link, an optical link, or a magneticlink. The programmer 60, or other similar external device, may also beused to couple power into the IPG for the purpose of operating the IPGor charging a replenishable power source, e.g., a rechargeable battery,within the IPG. Once the IPG 40 has been programmed, and its powersource has been fully charged or replenished, it may operate asprogrammed without the need for the external programmer 60 to bepresent.

[0029] Turning next to FIG. 2, there is shown a representativefunctional block diagram of one type of IPG 40 that may be used with aneurostimulation system. As seen in FIG. 2, the IPG 40 therein depictedis made up of a multiplicity of dual current sources 42. Each dualcurrent source 42 includes a positive current source, i.e., a currentsource that can function as an anode to “source” current to a load, anda current source that can function as a cathode to “sink” current from aload through the same node. The “load” is the tissue that residesbetween the two or more activated electrodes, and includes the wire (orother conductive element) and a coupling capacitor C that connects theelectrode to the common node of the dual current source.

[0030] Thus, for example, and as depicted in FIG. 2, a first dualcurrent source connected to electrode E1 of a first lead through acoupling capacitor C, may be programmed to produce a current of +I1 or−I1 through electrode E1, depending upon whether such dual currentsource is configured to operate as a cathode or an anode, when suchfirst dual current source is turned on or enabled. Similarly, a secondcurrent source, connected to electrode E2, when turned on or enabled,may be programmed to produce a current of +I2 or −I2 through electrodeE2. In a similar manner, a third current source, when enabled, may beprogrammed to produce a current of +I3 or −I3 through electrode E3. Annth current source, where n represents the number of electrodes on thefirst lead, is similarly connected to electrode En, and may beprogrammed to produce a current of +In or −In through electrode En whenturned on or enabled.

[0031] If a second lead, also having n electrodes, is positionedadjacent the first lead, each electrode is similarly connected to a dualcurrent source. For example, electrode E(n+1) is connected to a dualcurrent source that produces a current of +I(n+1) or −I(n+1) throughelectrode E(n+1) when such (n+1)th current source is enabled. In likemanner, all of the electrodes of the second lead are connected torespective dual current sources. There are thus 2n dual current sourcesthat are respectively connected to each of the 2n electrodes of thefirst and second leads (n electrodes on each lead). Alternativeembodiments (not shown) may employ less than 2n dual current sourcesconnected to 2n electrodes through a suitable multiplexer circuit.

[0032] A programmable current control circuit 44 is also provided withinthe IPG 40 that controls, i.e., turns on or enables, at specified times,a selected current source to operate as either a cathode or an anode tosource or sink a current having a desired amplitude. The control circuit44 also disables, or turns off, selected current sources, as controlledby programmed control data received from the external programmer, orotherwise resident within the IPG. The control circuit 44 furtherincludes the ability to measure the electrode voltage, E_(V1), EV_(V2),E_(V3), . . . E_(Vn), . . . E_(V(2n)), appearing at the output of eachdual current source 42, whether the electrode is activated ornon-activated. This effectively allows the electrode voltage, orelectric field at the electrode, to be measured, which in turnfacilitates impedance or field potential measurements to be made, whichmeasurements are used in carrying out various steps of the invention asdescribed below.

[0033] Thus, in operation, and as illustrated in FIG. 2, current controlcircuit 44 may turn on current sources +I1 and +I2 at the same time,i.e., during a time period T1, that current source −I(n+2) is turned on.All other current sources are turned off, or disabled, during the timeT1. Such action causes electrodes E1 and E2 to be activated as anodes atthe same time that electrode E(n+2) is activated as a cathode. That is,a current +I1 is “sourced” from electrode E1 and a current +I2 is“sourced” from electrode E2 at the same time that a current −I(n+2) is“sunk” into electrode E(n+2). The amplitudes of the currents +I1 and +I2may be any programmed values, and the amplitude of the current −I(n+2)should be equal to −(I1+I2). That is, the current that is sourced isequal to the current that is sunk.

[0034] After the time period T1, it is common to switch the polaritiesof the electrodes during a second time period T2. During T2, theelectrodes E1 and E2 are activated as cathodes, so that they both sinkcurrent, and electrode E(n+2) is activated as an anode, so that itsourches a current equal in amplitude to the current that is sunk byelectrodes E1 and E2. In this manner, a biphasic stimulation pulse 46 isproduced that is characterized by a first pulse (during time period T1)of one polarity, followed by a second pulse immediately or shortlythereafter (during time period T2) of the opposite polarity. Theelectrical charge associated with the first pulse is made so that it isequal to the charge associated with the second pulse, therebymaintaining charge balance during the stimulation. (Maintaining chargebalance when stimulating living tissue is generally considered animportant component of a stimulation regime.) Charge balance is commonlyachieved in a biphasic pulse 46 by making the amplitude of the firstpulse during time T1 equal to the amplitude of the second pulse duringtime period T2, where T1 equals T2. However, charge balance may also beachieved using other combinations of pulse duration and amplitude, e.g.,by making the amplitude of the second pulse equal to ½ the amplitude ofthe first pulse, while making the time period T2 equal to twice the timeperiod T1.

[0035] Next, with respect to FIG. 3, a functional block diagram ofanother type of IPG 40′ that may be used in a neurostimulation system isshown. The IPG 40′ shown in FIG. 3, includes a multiplicity of dualvoltage sources 42′, each being connected to one of the electrodes E1,E2, E3, . . . En, of a first lead, or to one of the electrodes E(n+1),E(n+2), . . . E(2n), of a second lead. Each dual voltage source 42′applies a programmed voltage, of one polarity or another, to itsrespective electrode, when enabled or turned on. For the configurationshown in FIG. 3, a separate dual voltage source 42′ is connected to eachelectrode node through a coupling capacitor C. (Other embodiments, notshown, may use one or two or more voltage sources that are selectivelyconnected to each electrode node through a multiplexer circuit.)

[0036] The control circuit 44′, or other circuitry within the IPG 40′,further includes the ability to measure the electrode current, E_(I1),E_(I2), E_(I3), . . . E_(In), . . . E_(I(2n)), flowing to or from itsrespective electrode, whether the electrode is activated ornon-activated, and the electrode voltage, E_(V1), E_(V2), E_(V3), . . .E_(Vn), . . . E_(V(2n)), appearing at the output of each non-activateddual voltage source 42′ These measurements facilitate impedance andelectric field measurements or calculations to be made, whichmeasurements are used in carrying out various steps of the invention asdescribed below.

[0037] A programmable voltage control circuit 44′ controls each of thedual voltage sources 42′, specifying the amplitude, polarity, andduration of the voltage that is applied to its respective terminal.Typically, stimulation is achieved by applying a biphasic stimulationpulse 46′ to the selected electrodes, wherein a voltage of a firstpolarity and amplitude is applied during time period T3, followed by avoltage of the appositive polarity and amplitude during time period T4.The biphasic stimulation pulse 46′ may be applied between any two ormore electrodes.

[0038] It should be noted that the functional block diagrams of FIGS. 2and 3 are functional diagrams only, and are not intended to be limiting.Those of skill in the art, given the descriptions presented herein,should be able to readily fashion numerous types of IPG circuits, orequivalent circuits, that carry out the functions indicated anddescribed, which functions include not only producing a stimulus currentor voltage on selected groups of electrodes, but also the ability tomeasure the voltage, or the current, flowing through an activated ornon-activated electrode. Such measurements allow impedance to bedetermined (used with a first embodiment of the invention) or allowelectric field potentials to be measured (used with a second embodimentof the invention), as described in more detail below. A preferred IPG isdescribed in international patent application WO 02/09808 A1 (publishedFeb. 7, 2002); and in U.S. patent application Ser. No. 09/626,010, filedJul. 26, 2000, which publication and application have been previouslyreferenced and are incorporated herein by reference.

[0039] With the descriptions of FIGS. 1-3 thus providing backgroundinformation relative to a neurostimulation system, the present inventionwill next be described. As has been indicated, the present inventionaddresses the problem of determining the relative position betweenelectrodes once the leads on which the electrodes are carried have beenimplanted. The present invention uses: (1) interelectrode impedance (onetechnique or embodiment) or (2) measured field potentials (anothertechnique or embodiment) to determine the relative orientation of oneelectrode on an implanted lead to other electrodes on the implanted leador adjacent implanted leads in the spinal column or other body/tissuelocation.

[0040] First, the interelectrode impedance technique of the inventionwill be explained in connection with FIGS. 4-6. The interelectrodeimpedance technique is performed by measuring impedance vectors. Avector is defined as an impedance value measured between two electrodesin the body. The value of the impedance vector is due primarily to twophysical entities:

[0041] (1) the electrode-electrolyte interface; and

[0042] (2) the bulk impedance between the electrodes.

[0043] The impedance tomography technique of the present inventionrelies upon the latter of the above two physical entities, i.e., uponthe bulk impedance between the electrodes.

[0044] The bulk impedance portion of the impedance vector may be furtherbroken up into two contributing factors: (a) the impedance of the tissueadjacent to the electrodes; and (b) the impedance of the tissue betweenthe electrodes.

[0045] The first factor (part a) makes up the majority of themeasurement, due to the higher and non-uniform current densities nearthe electrode surface. However, the second factor (part b), where thecurrent density is more uniform, has a roughly linear relationship todistance between the two electrodes, due to the definition ofresistance. Resistance, R, is defined as

R=(resistivity)×(distance)/cross-sectional area.

[0046] The second factor (part b) is used by the interelectrodeimpedance technique embodiment of the invention to determine therelative spacing between electrodes and to determine the relativeorientation of the leads.

[0047] By way of example, one first-order, simple embodiment of theinvention is as follows:

[0048] If two multipolar leads are placed in the spinal column, see FIG.5, each having four electrodes (the electrodes of one lead beingdesignated as e1, e2, e3, and e4; and the electrodes of the other leadbeing designated as E5, E6, E7 and E8), their relative orientation maybe inferred by making the following measurements:

[0049] 1. Monopolar impedances for all electrodes; and

[0050] 2. Bipolar impedances between each electrode on opposing leads.

[0051] The monopolar impedances are used to “correct” the bipolarimpedances for the first factor of bulk impedance, the strongly-weightedimpedance near the electrode. The corrected bipolar impedances are thenused to develop an impedance “map” between the electrodes. This mapreveals the relative orientation of the leads.

[0052] To illustrate, a sample correction formula is as follows:(distance  between  two  electrodes  e1  &  e2) ∝ (measured  bipolar  impedance  between  two  electrodes  e1  &  e2) + (2 * offset) − (monopolar  Z  for  electrode  e1) − (monopolar  Z  for  elctrode  e2),

[0053] where offset=an estimate of the impedance in the monopolarimpedance measurement that is NOT due to the tissue near the electrode.

[0054] After the bipolar impedances are corrected by the above formula,the relative orientation of the leads may be inferred by the relativeminima of the impedance values. Where the corrected bipolar impedancebetween two electrodes is a minimum relative to other electrodes on anopposing array, those electrodes are relatively adjacent. Thisinformation may then be loaded into a programmer, which can then providea graphic display of the assumed relative lead positions. Such dataand/or display might then be compared with previously measured orentered and stored graphics, indicating earlier orientations. Suchcomparison can thus help the physician/clinician to track the leadorientation to determine appropriate programming, reprogramming, or needfor surgical revision.

[0055] Also, for some programming systems, the present invention may beused to automatically setup the appropriate navigation tables forsteering multiple lead systems.

[0056]FIG. 4 illustrates data showing this simple embodiment applied todata from a patient with dual quadrapolar leads, which leads areoriented as depicted in FIG. 5. FIG. 6 shows the impedance map resultingfrom the measurements of FIG. 4. It can be seen that the impedance maps(FIG.6) correlate well to the orientation of the leads (FIG. 5).

[0057] The simple interelectrode impedance technique described above maybe enhanced by making more accurate corrections using the appropriatefield equations to calculate the monopolar and bipolar impedance of theelectrodes. Also, other geometric methods may be employed using theimproved “distance impedance” values to improve the mapping of theelectrode orientations.

[0058] Next, an alternative technique or embodiment for determiningrelative electrode positions for multipolar leads of a neurostimulationsystem will be described. Such alternative technique utilizes electricfield measurements of the implanted electrodes, and more particularly,electric field measurements on non-active electrodes caused byactivation of other electrodes. In a preferred embodiment of thisalternative embodiment, a constant current is sourced (anodes) and sunk(cathodes) from a predefined combination of electrodes. Such electrodesthus comprise the activated electrodes. Then, the resulting potentialsare measured at all other electrodes (those not involved in sourcing orsinking current), i.e., the non-activated electrodes. From thesemeasured potentials, the relative orientation of the electrodes, and theleads on which the electrodes are carried, may be determined.Advantageously, the use of field potentials represents an improvementover the use of impedance measurements, since the measured potentialvalues are less subject to the confounding effects of the tissueimpedance very close to the source/sink electrodes.

[0059] By way of example of this electric field potential measurementtechnique, consider FIGS. 7, 8 and 9. FIG. 7 represents the relativeposition of dual quadrapolar leads 21 and 31 after being implanted in apatient, as obtained using a fluoroscopic imaging device. In manyinstances, the necessary imaging equipment needed to obtain afluoroscopic image, such as is shown in FIG. 7, is not readilyavailable. Advantageously, the present electrical field potentialmeasurement technique represents an alternative approach to obtainingrelative electrode position information rather than using an expensiveand cumbersome imaging device.

[0060] Two combinations of anodes/cathodes are used to deliver currentto the leads of the dual quadrapolar leads 21 and 31. The firsttechnique is monopolar (current delivered or sourced from oneelectrode—the cathode—and sunk to the return electrode E_(R)—the anode).Thus, for each active monopolar combination, there are seven non-activeelectrodes on which the electric field may be measured. The secondtechnique is flanked tripolar stimulation (current delivered between twoanodes and one cathode, with the cathode being flanked on each side byan anode).

[0061] In both the monopolar stimulation and the tripolar stimulation, aconstant current is delivered to each electrode implanted in thepatient's body while the electric field potential is measured on allother electrodes NOT involved in sinking/sourcing current. The constantcurrent may be set to a subperception level, or to another suitablelevel that is comfortable for the patient.

[0062] The electric field potentials for the monopolar stimulation areplotted on the same chart in FIG. 8. The vertical axis is millivolts. Asseen in FIG. 8, the electrodes closest to the source electrode have ahigh potential (note: all plots in FIG. 8 and FIG. 9 are “negative”,i.e., more negative potentials results in more positive measured values,as shown in the plots). Thus, for example, consider electrode E8 (curve71), which has its highest potential relative to electrode E4, and itslowest potential relative to electrodes E1 and E2, and an intermediatepotential relative to electrode E3. This corresponds to the actualelectrode positions shown in FIG. 7, where electrode E8 is closest toelectrode E4, somewhat further from electrode E3, and farthest fromelectrodes E2 and E1. A similar analysis for the monopolar stimulationfields of the other electrodes reveals a similar relationship: theelectrodes closest to the source electrode have the higher potential.

[0063] The electric field potentials for the tripolar stimulation areplotted on the same chart in FIG. 9. Again, the vertical axis ismillivolts. As seen in FIG. 9, a better relative orientation can beobtained than can be obtained with the monopolar stimulation. Thoseelectrodes closest to the cathode have a high potential while thoseelectrodes closest to the anode have a lower potential relative to theelectrodes further away. For example, consider curve 72, which shows theelectric field potential of the non-active electrodes relative to thetripolar stimulation of electrodes E2E3E4, with E2 and E4 being anodes,and E3 being a cathode. As seen in FIG. 9, curve 72 has a peakcorresponding to electrode E7, which means electrode E7 is closest tothe cathode E3. Curve 72 further has lows or valleys corresponding toelectrodes E6 and E8, which means E6 and E8 are closest to anodeelectrodes E2 and E4. The actual orientation of the electrodes shown inFIG. 7 reveals that E6 is closest to E2, and E8 is closest to E4. Thus,it is seen that those electrodes closest to the flanked cathodicelectrode have a high potential while those electrodes closest to theanodic electrodes, on either side of the cathodic electrode, have alower potential relative to the electrodes further away.

[0064] Hence, it is seen that by measuring the potential field of thenon-active electrodes, when active electrodes are stimulated at constantcurrent levels, e.g., subperception levels, the relative orientation ofthe neurostimulation leads may be determined. Once known, the relativeorientation may be used to track lead migration, to setup stimulationconfigurations and parameters for nominal stimulation and/or navigation,and to automatically adjust stimulation energy to a previously-definedoptimal potential field in the case of lead migration or posturalchanges.

[0065] Next, with reference to FIG. 10, a flowchart is shown thatillustrates the main steps that may be used to carry out and apply theinvention described above in connection with FIGS. 7-9. As seen in FIG.10, a first step involves applying suitable stimuli, e.g., subperceptionstimuli, to a selected group of electrodes (block 82). Such applicationof stimuli defines the activated electrodes. While the stimuli are beingapplied to the activated electrodes, the electric field at thenon-activated electrodes is measured (block 84) and saved (block 86) aselectric field data (Block 86). Then, a determination is made as towhether there are other groups of electrodes that should be the“activated” electrodes, so that additional electric field measurementscan be made of the “non-activated” electrodes (block 85). If YES (thereare more electrode groups), then the steps shown at blocks 82, 84, and86 are repeated using the new group of activated electrodes. If NO (allthe electrode groups have been used), then the electric field data justobtained is compared to previously-saved electric field data for thesame non-activated electrodes (block 88).

[0066] The previously-saved electric field data may have been obtainedduring initial implantation of the leads, or during the last visit(several weeks or months ago) to the doctor. Or, the previously-savedelectric field data may have been obtained just a new hours or minutesago at a time when the patient's body had assumed a different postureposition. Regardless of when the previously-saved electric field datawas obtained, the purpose of the comparison performed at block 88 ofFIG. 10 is to determine if the relative position of the leads haschanged, which change in position would also have caused a relativechange in the position of the electrodes carried on the leads. Suchdetermination may be made by analyzing the electric field data (block90) as described above in connection with FIG. 8 and/or FIG. 9 todetermine whether the relative electrode orientation has changed.

[0067] The magnitude of the difference in the compared electric fielddata may advantageously provide a relative measure of how far the leadhas shifted or moved since the last electric field data was obtained.Advantageously, once a determination has been made that the leads(electrodes) have shifted relative to each other, appropriate correctionaction may be taken, as needed (block 92).

[0068] The corrective action taken at block 92 of FIG. 10 may include,for example, simply tracking the lead migration over time, so that othercorrective action, e.g., surgery to reposition the leads, can be takenwhen necessary. Even if new surgery to reposition the leads is notneeded, simply mapping the lead migration over time will enablereprogramming of the stimuli parameters as needed so that a desiredeffect can be obtained regardless of the lead movement.

[0069] The corrective action may further include setting up stimulationconfigurations and parameters for providing nominal stimulation suitablefor the electrodes in their new relative positions. For example, theamplitude of the stimulus applied to one electrode may be decreased ifit is determined that the electrode has migrated closer to anotherstimulating electrode of the same polarity during stimulation, therebypreserving approximately the same stimulation effect for the patient.Alternatively, the amplitude of the stimulus applied to the electrodemay be increased if the electrode has migrated closer to a stimulatingelectrode of the opposite polarity. Such amplitude adjustments may bemade manually or automatically, depending on the mode of operation ofthe neurostimulation system.

[0070] Yet another corrective action that may be taken at block 92 ofFIG. 10 is to adjust the distribution of the stimuli to a new locationthrough navigation. Navigation, as described in the previouslyreferenced patent documents, involves electronically shifting thestimulus current from one group of electrodes to another so as to shiftor move the location where the patient feels the most beneficialparesthesia, and/or receives the most benefit. Such navigation allowsthe neurostimulation system to be quickly “fitted” to a given patient.Fitting the neurostimulation system to the patient is necessary afterthe system is first implanted, and may also be necessary whenever theleads (electrodes) have moved. The present invention thus provides arelatively easy way to determine whether such lead movement hasoccurred, and thereby whether a refitting is or may be necessary.

[0071] Yet additional corrective action that may be taken at block 92 ofFIG. 10 in response to a determination that lead migration or posturalchanges have occurred includes manually or automatically adjusting thestimulation energy to a previously-defined optimal potential field.

[0072] It is thus seen that the present invention uses a measure ofimpedance or electric field to determine relative lead positions formultipolar leads in a multi-lead configuration of a neurostimulationsystem, e.g., a spinal cord stimulation system.

[0073] It is also seen that the invention uses impedance or electricfield measurements to determine relative lead positions, which impedanceor electric field measurements may be used as an automated or assistivemethod for setting up a programmer for navigation, other programming, ordiagnostic evaluations in spinal cord (or other neural) stimulation.

[0074] It is additionally seen that the invention may be directed to thestoring of impedance or electric field maps to chronically trackrelative lead positions in a programmer linked to a database, along withother patient data.

[0075] While the invention herein disclosed has been described by meansof specific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. A method for determining the relative leadposition of multipolar leads in a multi-lead configuration comprisingmeasuring impedance vectors between electrodes; and inferring therelative position of the lead by the relative minima and maxima of theimpedance values.
 2. The method of claim 1 wherein measuring theimpedance vectors comprises: measuring the monopolar impedances for allelectrodes; measuring bipolar impedances between each electrode onopposing leads; and correcting the bipolar impedances for adjacenttissue impedance.
 3. The method of claim 2 further including mapping thecorrected bipolar impedances to create an impedance map, which impedancemap reveals the relative orientation of the leads.
 4. The method ofclaim 3 further including storing the map for future reference.
 5. Themethod of claim 4 further including referring to the map at a futuretime to see if the lead position has changed, and if so, how much.
 6. Amethod for determining whether the relative position of electrodes usedby a neurostimulation system has changed, the method comprising stepsfor: (a) applying a stimulus to a selected group of the electrodes; (b)measuring the electric field at the electrodes not included in the groupof electrodes to which the stimulus is applied; (c) repeating steps (a)and (b) over time and determining if changes in the measured electricfield have occurred; (d) analyzing the changes in the electric field todetermine whether changes in the relative position of the electrodes hasoccurred.
 7. The method of claim 6 wherein step (a) comprises applying astimulus monopolarly between one selected electrode and a return orreference electrode.
 8. The method of claim 6 wherein step (a) comprisesapplying a stimulus tripolarly between three selected electrodes,wherein at least one of the three electrodes comprises an anode, and atleast one of the three electrodes comprises a cathode.
 9. The method ofclaim 8 wherein at least two of the three electrodes are configured asan anode, and one of the three electrodes is configured as a cathode.10. The method of claim 9 further including flanking the electrodeconfigured as a cathode with the electrodes configured as an anode,thereby creating a guarded cathode tripolar electrode configurationthrough which the stimulus is applied.
 11. The method of claim 6 furtherincluding storing the electric field measured at the electrodes notincluded in the group of electrodes through which the stimulus isapplied.
 12. The method of claim 6 wherein step (c) includes plottingthe measured electric field as a function of the selected electrodegroups through which the stimulus is applied.
 13. The method of claim 6further including initiating corrective action when a determination ismade that the relative position of the electrodes has changed.
 14. Themethod of claim 13 further including initiating the corrective actionautomatically.
 15. The method of claim 13 wherein the corrective actionincludes adjusting the amplitude of the stimulus applied to selectedelectrodes during operation of the neurostimulation system.
 16. Themethod of claim 13 wherein the corrective action includes changing theelectrodes through which a stimulus is applied during operation of theneurostimulation system.
 17. The method of claim 13 wherein thecorrective action includes adjusting the stimulation energy produced bythe neurostimulation system to a previously-defined optimal potentialfield.
 18. The method of claim 13 wherein the corrective actioncomprises setting up stimulation configurations and parameters thatprovide nominal stimulation suitable for the electrodes in their newrelative positions.
 19. A method for determining the relative positionof electrodes used by a neurostimulation system, the method comprisingsteps for: (a) applying a stimulus to a selected group of theelectrodes; (b) measuring the electric field potential at the electrodesnot included in the group of electrodes to which the stimulus isapplied; (c) analyzing the measured electric field potential at eachelectrode relative to the measured electric field potential at otherelectrodes to determine the position of the electrodes relative to eachother.
 20. The method of claim 19 wherein step (a) comprises applying astimulus monopolarly between one selected electrode and a return orreference electrode.
 21. The method of claim 19 wherein step (a)comprises applying a stimulus tripolarly between three selectedelectrodes, wherein at least one of the three electrodes comprises ananode, and at least one of the three electrodes comprises a cathode. 22.The method of claim 21 further including flanking the electrodeconfigured as a cathode with electrodes configured as an anode, therebycreating a guarded cathode tripolar electrode configuration throughwhich the stimulus is applied.
 23. The method of claim 19 wherein step(c) includes plotting the measured electric field as a function of theselected electrode groups through which the stimulus is applied.
 24. Anapparatus for determining the relative position or orientation ofelectrodes carried on neurostimulation leads, comprising: a multiplicityof implantable electrodes; means for applying a stimulus to a selectedgroup of the multiplicity of electrodes; means for measuring theelectric field at the electrodes not included in the group of electrodesto which the stimulus is applied; means for analyzing the measuredelectric field to determine the relative position or orientation of theelectrodes.
 25. The apparatus of claim 24 further including: means fordetermining if changes in the measured electric field have occurred overtime; and means for analyzing whether the changes that have occurred inthe electric field over time are indicative of a change in the relativeposition or orientation of the electrodes.
 26. The apparatus of claim 25further including means for automatically initiating corrective actionin the event that a change in the relative position or orientation ofthe leads has occurred.